The objectives of this study are to define the mechanism(s) responsible for the hypolipidemic effects of dietary polyunsaturated fats in both normal subjects and patients with type IIa, IIb, and IV hyperlipidemia. We will also explore the plasma cholesterol elevating effect of saturated fat in the diet. During three dietary periods, we will compare the effects of isocaloric substitution of both polyunsaturated and saturated fats for monounsaturated fat in the diet upon the plasma lipids, lipoproteins and apoproteins and upon other aspects of lipid metabolism. Specifically, we intend to quantitate the sterol balance (fecal neutral steroids and bile acids), the lipid composition of bile, the turnover of 125I low density lipoproteins (LDL), the activity of post heparin hepatic and lipoprotein lipase and the changes in individual apoproteins during each dietary period and to compare the results obtained in both normal and hyperlipidemic subjects. The synthesis of cholesterol and phospholipid by mononuclear cells will be assayed after their removal from the plasma by an in vitro system. The fatty acid content of plasma and biliary lipids will be monitored for each dietary period. These combined assessments in the same patient, should provide for an evaluation of the actions of polyunsaturated and saturated fat.